OAuth2 and SPA's

I’m not too familiar with OAuth2 and would like some advice on implementing access control to Restful web services that are used by SPA’s (single page apps).

I’d like to use a CMS as a single sign on entry point where users login/register via the CMS and are therefore authenticated via a session cookie in the normal way.

They would then be shown a menu with links to CMS pages that are actually entry points to SPA’s - they load the various JS resources etc.

A page that loads an SPA will have access control via the normal mechanisms of the CMS but the web services behind it will be built with Yii2.

I have a plugin for the CMS that allows management of OAuth2 providers - params: key, secret, redirect url / end points: oauth/authorize, oauth/request_token, oauth/request_access - but I don’t really understand how all this can be used to provide authentication to the web services.

Can anyone help me shortcut my understanding of OAuth2 in this scenario?